Get an app called "air video"
But I was thinking more of something that can stream JUST one app's window to the ATV, sort of, wanting to stream XMBC for instance.
Most apps I encounter either airplay the whole screen (useless) or will stream something by themselves (video, etc).
Probably such thing doesn't exist.
Edit:
Lemme rephrase that not to sound so off topic.
The only reason I would like to jailbreak the ATV would be to install XMBC (and there is another app, 8 bucks or something) so that it can connect to my Mac (which is on 24/7 mostly, and is my main computer) and browse (via the remote) the library of whatever I have and pick something and stream it (at least, that is what I think XMBC does, otherwise why the hype).
So far I can do that via iTunes (mac, ATV, remote) but then iTunes sucks, it expects you to have videos on the format it handles and if I want subtitles for things NOT bought on the AppStore I need to hard-encode them (hello again 2001).
So, even if I don't know if it would work, I think by jail breaking you can install XMBC, then use it's wonderful interface to browse whatever is on your Mac's XMBC collection (with -I presume- info automatically fetched from the intrawebs and whatnot) and then stream something, using the remote, while having your PC on the other room working on other things.
So far I can either stream the whole Lion screen, or have to use an iDevice as either an intermediary or standalone streamer, but then there is no interaction with the remote, no picking of a video, no nothing. In short, no collection, just "what do you wanna play today Sir" and then going and copying it to the iDevice or being limited to things (VLCRemote and others).